1. Field of the Invention
The present invention relates to a thermal printer.
2. Description of the Related Art
As a thermal printer, there is known a structure including a line thermal head (hereinafter, simply referred to as a thermal head) and a platen roller to be in contact with the thermal head that are assembled in a manner releasable (separation type). In such a separation type thermal printer, for example, a head unit including a thermal head is mounted on a side of a case that houses roll paper, and a platen unit including the platen roller is mounted on a side of a printer cover connected to the case in such a manner that the platen roller can be opened/closed. According to this structure, the thermal head and the platen roller can be assembled in a manner releasable along with open/close operation of the printer cover. Therefore, workability upon setting of recording paper, upon paper jam handling, upon maintenance, replacement, or the like of the thermal head, the platen roller, or the like can be improved comparing to a structure including a head unit and a platen unit that are integrally assembled in a case (integrated type).
The above-described separation type thermal printer generally includes a lock mechanism that detachably holds the platen roller when the printer cover is closed. Thus, the thermal printer is generally structured in such a manner that the platen roller is assembled with the thermal head while the whole platen roller is in uniform pressure contact with the thermal head.
However, practically, there is some component precision error, assembly error, or the like. Thus, when the printer cover is closed, failure such as insufficient lock of the platen roller, impossible lock itself, and non-uniform contact of the whole platen roller with the thermal head is occasionally caused even when lock is possible. Therefore, measures to cancel component precision error, assembly error, or the like are thought of.
As one example of conventional measures, there is known a structure including a pair of locking pins provided on a case side, and a pair of catches, which is provided on a printer cover side and to which the respective locking pins are locked so as to be interlocked with each other, wherein one of the catches has a contour enlarged portion having a size equivalent to a displacement amount of the printer cover due to looseness.
However, the above-described conventional technique still has a following issue. For open operation of a printer cover, operation of a releasing part (such as a release lever and a release button) that releases a lock mechanism is generally required, enabling to restrict users to perform common operation. On the other hand, for close operation, users can perform pressing operation at any position of the printer cover, allowing different operation positions depending on users. For example, pressing operation may be performed on a part of the printer cover at a position corresponding to a central part in a longitudinal direction of a platen roller mounted on the printer cover, or may be performed on a part thereof at a position corresponding to an end part of the platen roller, for example. That is, operation positions are not limited to a single position.
When a user performs pressing operation on a portion of the printer cover at a position corresponding to one end of the platen roller (presses one side of the printer cover), the pressing force concentrates on one bearing member side mounted on the platen shaft. That is, pressing force on the printer cover varies due to influence of operation positions of the printer cover, and thus the platen unit occasionally moves to obliquely incline. As a result, failure of so-called one-side lock where only the one bearing member side is locked and the other bearing member side is not locked or sufficiently locked is occasionally caused. When one-side lock is caused, the platen roller does not come into contact with the thermal head with uniform and appropriate contact pressure, causing defective printing and degradation of printing quality, and also causing occasional paper jam or the like.